Motor

Abstract

A small DC motor includes a motor frame having a cylindrical portion, the cylindrical portion having a constant thickness and having a cross section in a shape that comprises four sides and connecting portions, each of the connecting portions connecting adjacent two of the four sides and being located inward from a corresponding corner in a quadrangle comprising the four sides; and a magnet having a circumferential surface on an inside thereof and having conformable contact with the motor frame on an outside thereof.

What is claimed is: 
     
       1. A motor comprising: a motor frame being a magnetic material and being a magnetic constituent portion of the motor, wherein the motor frame comprises four straight portions and four connecting portions, each of the connecting portions is located between adjacent two of the four straight portions; a magnet disposed on an inner surface of the motor frame, wherein the magnet include four magnetic poles and has an inner surface forming a circular opening; a rotor having a shaft and rotatably disposed in the circular opening of the magnet; an end cap attached to the motor frame; a commutator provided on the shaft; and an air gap provided between the inner surface of the magnet and the rotor, wherein each magnetic pole center of the magnet is corresponded to the connecting portions of the motor frame, and wherein each of the connecting portions of the motor frame has an arc shape and is a segment of a circle concentric with a center of the shaft. 
     
     
       2. A motor according to  claim 1 , wherein the magnet has a maximum thickness at portions corresponding to the connecting portions of the motor frame and wherein the thickness of the magnet gradually decreases from the maximum thickness to a minimum thickness at portions corresponding to the straight portions of the motor frame. 
     
     
       3. A motor according to  claim 2 , wherein a ratio of the width of the maximum thickness portion to the width of the minimum thickness portion in the radial direction of the magnet is 2.0 or more. 
     
     
       4. A motor according to  claim 3 , wherein the ratio of the width of the maximum thickness portion to the width of the minimum thickness portion in the radial direction of the magnet is between 2.0 to 4.0. 
     
     
       5. A motor comprising: a motor frame being a magnetic material and being a magnetic constituent portion of the motor, wherein the motor frame comprises straight portions and connecting portions, each of the connecting portions being connected to adjacent two of the straight portions; a rotor having a shaft and rotatably disposed inside said motor frame; a commutator provided on the shaft; and a magnet disposed between said rotor and said motor frame, having an inner surface and an outer surface opposite to the inner surface, wherein a radial thickness between the inner surface and the outer surface is variable in a circumferential direction and wherein the inner surface of said magnet surrounds said rotor, and wherein the motor frame is configured so that when the motor frame is viewed in a cross-sectional view resulting from a sectional cut taken along a plane perpendicular to the axis of rotation of the rotor the connecting portions are located inside of a hypothetical rectangle which could be formed by extending the length of each straight portion to intersect with another straight portion. 
     
     
       6. A motor according to  claim 5 , wherein each of the connecting portions of said motor frame has an arc shape. 
     
     
       7. A motor according to  claim 6 , wherein each of the connecting portions of said motor frame is a segment of a circle concentric with a center of a rotation axis of said rotor. 
     
     
       8. A motor according to  claim 7 , wherein the inner and outer surfaces of said magnet corresponding to the connecting portions have curved surfaces and wherein a radius of curvature of the curved surface of the inner surface is different from a radius of curvature of the curved surface of the outer surface. 
     
     
       9. A motor according to  claim 7 , wherein said magnet includes a minimum thickness portion having a minimum radial thickness and a maximum thickness portion having a maximum radial thickness and wherein the minimum thickness portion corresponds to the straight portion and the maximum thickness portion corresponds to the connecting portion. 
     
     
       10. A motor according to  claim 9 , wherein the inner and outer surfaces of said magnet corresponding to the connecting portions have curved surfaces and wherein a radius of curvature of the curved surface of the inner surface is different from a radius of curvature of the curved surface of the outer surface.